Sheathed-element glow plugs of this type are well-known. These are used for starting self-igniting combustion engines (diesel engines). It is known that the self-igniting combustion process requires initial ignition. To this end, sheathed-element glow plugs are used, which are sealingly mounted in the wall of a combustion chamber (in the case of a combustion engine, a cylinder chamber) in such manner, that a heating element extends into the combustion chamber. In this connection, the heating element is in contact with a fuel-air mixture to be ignited.
It is known that one can use ceramic heating elements, whose glowing segment is made of a ceramic, electrically conductive material. These are distinguished by a high rigidity and a high resistance to the atmosphere prevailing in the combustion chamber. In addition, ceramic heating elements are resistant to high temperatures.
In order to start the self-igniting combustion engine, the heating element is connected to a voltage source (normally an automotive battery in motor vehicles). A current, which causes the glowing segment of the heating element to heat up, flows as a function of the electrical resistance of the heating element.
In order to rapidly heat the tip of the heating element, it is known that, in the region of the tip of the heating element, one may locally provide a ceramic material that has a higher specific electrical resistance than the rest of the heating-element body. This concentrates the electrical resistance of the heating element in the heating-element tip, so that it locally heats up in a more rapid and intense manner. In this case, it is disadvantageous that such heating elements, which have different materials exhibiting different specific electrical resistances, are difficult and costly to manufacture.
German Published Patent Application No. 195 06 950 describes a sheathed-element glow plug, in which the electrically conductive cross-section is reduced in the region of a heating-element tip. This reduction in the electrically conductive cross-section causes the heating element to heat up more intensely here than in the rest of it. The electrically conductive cross-section is reduced by providing the sheathed-element glow plug with bore holes, which are subsequently filled up with an electrically insulating material. In this connection, it is disadvantageous that such a reduction in the cross-section may only be attained in a costly manner, using additional manufacturing-method steps. In particular, when electrically insulating materials are introduced in the region of the sheathed-element glow plug experiencing the most heating, the different thermal expansion coefficients of the materials utilized can cause mechanical stresses to build up, which may result in damage to or the destruction of the sheathed-element glow plug.